Multiprocessor computer system with sectored cache line mechanism for cache intervention

ABSTRACT

A method of maintaining coherency in a multiprocessor computer system wherein each processing unit&#39;s cache has sectored cache lines. A first cache coherency state is assigned to one of the sectors of a particular cache line, and a second cache coherency state, different from the first cache coherency state, is assigned to the overall cache line while maintaining the first cache coherency state for the first sector. The first cache coherency state may provide an indication that the first sector contains a valid value which is not shared with any other cache (i.e., an exclusive or modified state), and the second cache coherency state may provide an indication that at least one of the sectors in the cache line contains a valid value which is shared with at least one other cache (a shared, recently-read, or tagged state). Other coherency states may be applied to other sectors in the same cache line. Partial intervention may be achieved by issuing a request to retrieve an entire cache line, and sourcing only a first sector of the cache line in response to the request. A second sector of the same cache line may be sourced from a third cache. Other sectors may also be sourced from a system memory device of the computer system as well. Appropriate system bus codes-are utilized to transmit cache operations to the system bus and indicate which sectors of the cache line are targets of the cache operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to the following applications filedconcurrently with this application, each of which is herebyincorporated: U.S. patent application Ser. No. 09/753,057; and U.S.patent application Ser. No. 09/752,862.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to computer systems,specifically to multiprocessor computer systems having caches whichshare memory and are thus required to maintain cache coherency, and moreparticularly to an improved method of maintaining cache coherency in acache architecture having sectored cache lines.

2. Description of Related Art

The basic structure of a conventional multiprocessor computer system 10is shown in FIG. 1. Computer system 10 has several processing units, twoof which 12 a and 12 b are depicted, which are connected to variousperipheral devices, including input/output (I/O) devices 14 (such as adisplay monitor, keyboard, graphical pointer (mouse), and a permanentstorage device or hard disk), memory device 16 (such as random accessmemory or RAM) that is used by the processing units to carry out programinstructions, and firmware 18 whose primary purpose is to seek out andload an operating system from one of the peripherals (usually thepermanent memory device) whenever the computer is first turned on.Processing units 12 a and 12 b communicate with the peripheral devicesby various means, including a generalized interconnect or bus 20, ordirect memory access channels (not shown). Computer system 10 may havemany additional components which are not shown, such as serial,parallel, and universal system bus (USB) ports for connection to, e.g.,modems, printers or scanners. There are other components that might beused in conjunction with those shown in the block diagram of FIG. 1; forexample, a display adapter might be used to control a video displaymonitor, a memory controller can be used to access memory 16, etc. Thecomputer can also have more than two processing units.

In a symmetric multi-processor (SMP) computer, all of the processingunits are generally identical, that is, they all use a common set orsubset of instructions and protocols to operate, and generally have thesame architecture. A typical architecture is shown in FIG. 1. Aprocessing unit includes a processor core 22 having a plurality ofregisters and execution units, which carry out program instructions inorder to operate the computer. An exemplary processing unit includes thePowerPC™ processor marketed by International Business Machines Corp. Theprocessing unit can also have one or more caches, such as an instructioncache 24 and a data cache 26, which are implemented using high speedmemory devices. Caches are commonly used to temporarily store valuesthat might be repeatedly accessed by a processor, in order to speed upprocessing by avoiding the additional latency of loading the values frommemory 16. These caches are referred to as “on-board” when they areintegrally packaged with the processor core on a single integrated chip28. Each cache is associated with a cache controller (not shown) thatmanages the transfer of data and instructions between the processor coreand the cache memory.

A processing unit can include additional caches, such as cache 30, whichis referred to as a level 2 (L2) cache since it supports the on-board(level 1) caches 24 and 26. In other words, cache 30 acts as anintermediary between memory 16 and the on-board caches, and can store amuch larger amount of information (instructions and data) than theon-board caches can, but at a longer access penalty. For example, cache30 may be a chip having a storage capacity of 512 kilobytes, while theprocessor may be an IBM PowerPC™ 604-series processor having on-boardcaches with 64 kilobytes of total storage. Cache 30 is connected to bus20, and all loading of information from memory 16 into processor core 22must come through cache 30. Although FIG. 1 depicts only a two-levelcache hierarchy, multi-level cache hierarchies can be provided wherethere are many levels (L3, L4, etc.) of serially connected caches.

In a multi-level cache, if a copy of a value is in every level of thecache, the cache hierarchy is referred to as being “inclusive.” It isnot necessary, however, to keep a copy of each value in the lowerlevels, and an inclusivity bit field may be added to the caches toindicate whether or not the cache is inclusive. For example, athree-level cache structure might provide an L3 cache which was notinclusive, such that a value residing in the L2 cache might not bepresent in the L3 cache. In this example, if an L2 cache issues a readcommand for a value that is not present in any of the caches of thatprocessing unit, it can be passed to that L2 cache without (necessarily)loading it into the L3 cache.

In an SMP computer, it is important to provide a coherent memory system,that is, to cause write operations to each individual memory location tobe serialized in some order for all processors. By way of example,assume a location in memory is modified by a sequence of writeoperations to take on the values: 1, 2, 3, 4. In a cache coherentsystem, all processors will observe the writes to a given location totake place in the order shown. However, it is possible for a processingelement to miss a write to the memory location. A given processingelement reading the memory location could see the sequence 1, 3, 4,missing the update to the value 2. A system that implements theseproperties is said to be “coherent”. Nearly all coherency protocolsoperate only to the granularity of the size of a cache block. That is tosay, the coherency protocol controls the movement of and writepermissions for operand data or instructions on a cache block basis, andnot separately for each individual memory location.

There are a number of protocols and techniques for achieving cachecoherence that are known to those skilled in the art. All of thesemechanisms for maintaining coherency require that the protocols allowonly one processor to have a “permission” that allows a write operationto a given memory location (cache block) at any given point in time. Asa consequence of this requirement, whenever a processing elementattempts to write to a memory location, it must first inform all otherprocessing elements of its desire to write the location and receivepermission from all other processing elements to carry out the write.

To implement cache coherency in a system, the processors communicateover a common generalized interconnect (i.e., bus 20). The processorspass messages over the interconnect indicating their desire to read fromor write to memory locations. When an operation is placed on theinterconnect, all of the other processors “snoop” (monitor) thisoperation and decide if the state of their caches can allow therequested operation to proceed and, if so, under what conditions. Thereare several bus transactions that require snooping and follow-up actionto honor the bus transactions and maintain memory coherency. Thesnooping operation is triggered by the receipt of a qualified snooprequest, generated by the assertion of certain bus signals. Instructionprocessing is interrupted only when a snoop hit occurs and the snoopstate machine determines that an additional cache snoop is required toresolve the coherency of the offended sector.

This communication is necessary because, in systems with caches, themost recent valid copy of a given block of memory may have moved fromthe system memory 16 to one or more of the caches in the system (asmentioned above). If a processor (say 12 a) attempts to access a memorylocation not present within its cache hierarchy, the correct version ofthe block, which contains the actual (current) value for the memorylocation, may either be in the system memory 16 or in one of more of thecaches in another processing unit, e.g. processing unit 12 b. If thecorrect version is in one or more of the other caches in the system, itis necessary to obtain the correct value from the cache(s) in the systeminstead of system memory.

For example, consider a processor, say 12 a, attempting to read alocation in memory. It first polls its own L1 cache (24 or 26). If theblock is not present in the L1 cache, the request is forwarded to the L2cache (30). If the block is not present in the L2 cache, the request isforwarded on to lower cache levels, e.g., the L3 cache. If the block isnot present in the lower level caches, the request is then presented onthe generalized interconnect (20) to be serviced.

Once an operation has been placed on the generalized interconnect, allother processing units snoop the operation and determine if the block ispresent in their caches. If a given processing unit has the blockrequested by processing unit in its L1 cache, and the value in thatblock is modified, and any lower level caches also have copies of theblock, then their copies are stale, since the copy in the processor'scache is modified. Therefore, when the lowest level cache (e.g., L3) ofthe processing unit snoops the read operation, it will determine thatthe block requested is present and modified in a higher level cache.When this occurs with an in-line cache structure, the L3 cache places amessage on the generalized interconnect informing the processing unitthat it must “retry” it's operation again at a later time, because theactual value of the memory location is in the L1 cache at the top of thememory hierarchy and must be retrieved to make it available to servicethe read request of the initiating processing unit.

Once the request from an initiating processing unit has been retried,the L3 cache begins a process to retrieve the modified value from the L1cache and make it available at the L3 cache, main memory or both,depending on the exact details of the implementation. To retrieve theblock from the higher level caches, the L3 cache sends messages throughthe inter-cache connections to the higher level caches, requesting thatthe block be retrieved. These messages propagate up the processing unithierarchy until they reach the L1 cache and cause the block to be moveddown the hierarchy to the lowest level (L3 or main memory) to be able toservice the request from the initiating processing unit.

The initiating processing unit eventually re-presents the read requeston the generalized interconnect. At this point, however, the modifiedvalue has been retrieved from the L1 cache of a processing unit andplaced into system memory, and the read request from the initiatingprocessor will be satisfied. The scenario just described is commonlyreferred to as a “snoop push”. A read request is snooped on thegeneralized interconnect which causes the processing unit to “push” theblock to the bottom of the hierarchy to satisfy the read request made bythe initiating processing unit.

Thus, when a processor wishes to read or write a block, it mustcommunicate that desire with the other processing units in the system inorder to maintain cache coherence. To achieve this, the cache coherenceprotocol associates with each block in each level of the cachehierarchy, a status indicator indicating the current “state” of theblock. The state information is used to-allow certain optimizations inthe coherency protocol that reduce message traffic on the generalizedinterconnect and the inter-cache connections. As one example of thismechanism, when a processing unit executes a read it receives a messageindicating whether or not the read must be retried (i.e., reissuedlater). If the read operation is not retried, the message usually alsoincludes information allowing the processing unit to determine if anyother processing unit also has a still active copy of the block (this isaccomplished by having the other lowest level caches give a “shared” or“not shared” indication for any read they do not retry). Therefore, aprocessing unit can determine whether any other processor in the systemhas a copy of the block. If no other processing unit has an active copyof the block, the reading processing unit marks the state of the blockas “exclusive”. If a block is marked exclusive it is permissible toallow the processing unit to later write to the block without firstcommunicating with other processing units in the system because no otherprocessing unit has a copy of the block. Therefore, it is possible for aprocessor to read or write a location without first communicating thisintention onto the interconnection, but only where the coherencyprotocol has ensured that no other processor has an interest in theblock.

The foregoing cache coherency technique is implemented in a specificprotocol referred to as “MESI.” In this protocol, a cache block can bein one of four states, “M” (Modified), “E” (Exclusive), “S” (Shared) or“I” (Invalid). Under the MESI protocol, each cache entry (e.g., a32-byte sector) has two additional bits which indicate the state of theentry, out of the four possible states. Depending upon the initial stateof the entry and the type of access sought by the requesting processor,the state may be changed, and a particular state is set for the entry inthe requesting processor's cache. For example, when a sector is in theModified state, the addressed sector is valid only in the cache havingthe modified sector, and the modified value has not been written back tosystem memory. When a sector is Exclusive, it is present only in thenoted sector, and is consistent with system memory. If a sector isShared, it is valid in that cache and in at least one other cache, allof the shared sectors being consistent with system memory. Finally, whena sector is Invalid, it indicates that the addressed sector is notresident in the cache.

A further improvement in accessing cache blocks can be achieved usingthe cache coherency protocol. This improvement, referred to as“intervention,” allows a cache having control over a memory block toprovide the data in that block directly to another cache requesting thevalue (for a read-type operation), in other words, bypassing the need towrite the data to system memory and then have the requesting processorread it back again from memory. Intervention can generally be performedonly by a cache having the value in a block whose state is Modified orExclusive. In both of these states, there is only one cache block thathas a valid copy of the value, so it is a simple matter to source(write) the value over the bus 20 without the necessity of first writingit to system memory. The intervention procedure thus speeds upprocessing by avoiding the longer process of writing to and reading fromsystem memory (which actually involves three bus operations and twomemory operations). This procedure not only results in better latency,but also increased bus bandwidth.

There are many variations of the MESI protocol. The tagged (“T”) stateis used to identify a cache block which is inconsistent with systemmemory (i.e., modified) and is further responsible for writing thecorrect (current) value to memory upon deallocation (or to pass on thetag to another cache block during intervention). The T state can be usedto share a modified value, by marking one of the sharing blocks as(temporarily) responsible for maintaining a valid copy of the value. Therecently read (“R”) state can be used to allow intervention when thevalue is unmodified but shared among many caches, so as to convenientlymark a single one of the sharing caches as being responsible forintervention. The hover (“H”) state allows a cache line to maintain anaddress in the directory even though the corresponding value in thecache entry array is an invalid copy, so that it can snoop the correctvalue for its processing unit if the value happens to be broadcast aspart of an intervention between the caches of two other processingunits.

While the foregoing techniques are very useful in facilitating shareduse of the system memory among the various caches, there are stillseveral inefficiencies in these designs, particularly for sectoredcaches. For example, a 128-byte cache line may be divided into four32-byte sectors, with each of the four sectors containing valid data. Ifa store operation writes new data to one of the sectors, the entirecache line must be invalidated, even though three of the four sectors inthe line are still valid. Thereafter, if the processing unit associatedwith that cache issues a request to read one of the three valid sectors,the entire cache line must be obtained from elsewhere in the memoryhierarchy (either from system memory, or from another cache viaintervention), even though the valid data is already present in thecache. Although separate coherency bits could be provided for each ofthe sectors, this approach would effectively remove the benefits thatare otherwise achieved from sectoring.

In light of the foregoing, it would be desirable to devise an improvedmethod of handling cache transactions which did not require theinvalidation of otherwise valid sectors in cache line. It would befurther advantageous if the method could also provide for more efficientcache intervention.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved method of maintaining cache coherency in a multiprocessorsystem.

It is another object of the present invention to provide such a methodwhich does not require invalidation of portions of a sectored cache linewhen one sector becomes invalid.

It is yet another object of the present invention to provide such amethod which allows for partial intervention of requested data, that is,intervention by transmitting less than the entire cache line.

The foregoing objects are achieved in a method of maintaining coherencyamong a plurality of caches each associated with a respective processingunit of a multiprocessor computer system, wherein each of the caches hasa plurality of cache lines, and each of the cache lines is divided intoa plurality of sectors, the sectors having a smaller granularity thanthe cache lines, the method generally comprising the steps of assigninga first cache coherency state to a first sector of a cache line of oneof the caches, and assigning a second cache coherency state, differentfrom the first cache coherency state, to the cache line whilemaintaining the first cache coherency state for the first sector of thecache line. For example, the first cache coherency state may provide anindication that the first sector contains a valid value which is notshared with any other cache (i.e., an exclusive or modified state).Also, the second cache coherency state may provide an indication that atleast one of the sectors in the cache line contains a valid value whichis shared with at least one other cache (a shared, recently-read, ortagged state). Other coherency states may be applied to other sectors inthe same cache line. Partial intervention may be achieved by issuing arequest to retrieve an entire cache line, and sourcing only a firstsector of the cache line in response to the request. A second sector ofthe same cache line may be sourced from a third cache. Other sectors mayalso be sourced from a system memory device of the computer system aswell. Appropriate system bus codes are utilized to transmit cacheoperations to the system bus and indicate which sectors of the cacheline are targets of the cache operation.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a block diagram of a prior art multiprocessor computer system;

FIGS. 2A and 2B is a block diagram of one embodiment of a processingunit of a multiprocessor computer system constructed in accordance withthe present invention, depicting the use of sectored cache lines havingseparate cache coherency states for each sector; and

FIG. 3 is a timing diagram of various system bus signals which areutilized to implement a cache coherency protocol in accordance with thepresent invention wherein different coherency states can have differentgranularities.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

With reference now to the figures, and in particular with reference toFIG. 2, there is depicted one embodiment 40 of a processing unit of amultiprocessor computer system constructed in accordance with thepresent invention. While the present invention is directed to a methodof handling cache operations in a memory-coherent, multiprocessor systemsuch as the system of FIG. 2, the present invention could be applied tocomputer systems that have additional hardware components not shown inFIG. 2, or having a different interconnection architecture (or both), sothose skilled in the art will appreciate that the present invention isnot limited to the generalized system shown in that figure.

Processing unit 40 is primarily comprised of a processor core 42 and L2cache 44. Core 42 includes an L1 cache in this depiction, although it isnot shown separately. L2 cache 44 includes a 4-way set associative L2directory 46, an L2 cache entry array 48, one or more snooper machines50, and one or more read/claim (RC) machines 52. Core 42 communicateswith L2 cache 44 via load and store ports 54 and 56, provided as part ofthe load/store unit (LSU) of core 42. These address ports are providedas inputs to a multiplexor 58 whose output is fed to several tagcomparators 60 used by L2 directory 46 to determine whether an accessrequest “hits” or “misses” cache 44 (i.e., whether the requested addressis contained in the L2 cache). In this example, 44 bits of the addressaddr(20:63) are included in the output of multiplexor 58, but only 28bits addr(20:47) are fed to comparators 60. The other inputs tocomparators 60 come from the address tags of the four members of indexline 62 in the 4-way set associative L2 directory 46 where each member'saddress tag represents address(20:47) of the cache line residing in theL2 cache 48.

Each index line in the 4-way set associative L2 directory 46 correspondsto four 128B cache lines in cache entry array 48, e.g., the fourdirectory entries in index line 62 corresponds to four 128B cache lines64. The present invention is directed to a cache architecture in whichthe cache lines are sectored. Cache sectoring is generally known in theart, but the present invention provides a novel approach to cachecoherency for a sectored cache, as explained further below. In thedepicted embodiment, each cache line is 128 bytes long, with foursectors of 32 bytes each. Thus, each of the four cache lines 64 has foursectors per cache line, and index line 62 has four segments for each ofthe four lines, corresponding respectively to each of the four sectorsof the four cache lines 64. Each input to comparators 60 from L2directory 46 comes from a different one of these segments of index line62. The outputs of comparators 60 are combined in a logical OR fashionto indicate a hit. When a hit is indicated, the particular comparator 60having the high output is used to identify the hit vector which pointsto the appropriate cache line in the four 128B lines 64. This hit vectorcontrols the selection for another multiplexor 66 whose output providesthe L2 cache value (operand data or program instruction) to core 42.This hit vector also controls the selection of multiplexer 66B whichprovides to RC 52 the state information for each of the four 32B sectorsof the 128B line hit.

In addition to examining the address information in L2 directory 46, itis also necessary to examine coherency state information concerning eachof the sectors in order to determine if a valid copy of the requestedvalue is present in cache 46. It may be that a previous version of thevalue was stored in cache 44, but an operation has overwritten thatvalue in another processing unit of the multiprocessor computer system,such that the old copy in L2 cache entry array 48 is stale, that is,possibly incorrect. The present invention uses cache coherency statessuch as the modified, exclusive, shared and invalid states of the priorart MESI cache coherency protocol, with one important difference—thepresent invention applies the various coherency states to differentgranularities depending upon the particular state.

In a preferred implementation of a cache coherency protocol according tothe present invention, the exclusive and modified states are appliedonly at the sector (core) granularity (e.g., 32 bytes), while theinvalid and shared states may be applied at the overall cache line(memory) granularity (e.g., 128 bytes), or at the sector granularity.

Other coherency states may be used, such as the “R” (recently read)state described in the Background section. The “R” state may be appliedto the overall cache line granularity as well.

Use of different granularities for different coherency states offers acombination of benefits which have previously been unobtainable. Forexample, in the simplest case wherein a particular requested value ispresent (valid) in cache 44, it is not necessary that the other threesectors in that cache line all be valid. In the prior art, a storeoperation from an adjacent processing unit to a different sector in thesame cache line would result in the invalidation of the entire cacheline. This result would thus require the entire cache line to beretrieved from the remainder of the memory hierarchy, even though theparticular sector requested was actually valid in the cache, therebyincreasing latency. With the present invention, this inefficiency isavoided. The state information for each of the four sectors of the cacheline is passed to RC machine 52 and, when a hit is indicated on a loadoperation, the hit vector is used to check the coherency state for theparticular sector. If the state for that sector is valid (in theexemplary implementation, exclusive, modified, recently read, orshared), then the L2 cache value is passed to a core interface unit(CIU—not shown) in core 42, regardless of the states of the othersectors.

In addition to the coherency states that are assigned to each individualsector, the overall cache line can have a coherency state as well, forthe above-noted states. For example, the overall cache line may bemarked as recently read (“R”), even though some sectors are invalid.

If the address requested by core 42 is not present (valid) in cache 44,the value must be retrieved from elsewhere in the memory hierarchy, andloaded into cache 44. Retrieval is accomplished using RC machine 52which communicates with the system bus 68, as explained further below.If the block of memory corresponding to the cache line for the requestedvalue is not present in cache 44, then a cache line in L2 cache entryarray 48 must be made available. If the cache is full, a conventionaltechnique may be used to evict a cache line, such as a least-recentlyused (LRU) algorithm. This invention uses a 4-way set associativedirectory in its example but it should be apparent to one skilled in theart that this invention could be easily applied to other directorystructures (e.g. direct mapped directory)

Additional benefits of the present invention relating to both load andstore operations may be gleaned from the timing diagram of FIG. 3. Fourbuses are shown, a request bus 70, a snoop response bus 72, a combinedresponse bus 74, and a data bus 76. Request bus 70 has three signals.Request signal 78 indicates that a valid request is being presented onsystem bus 68. Address/tag/type signal 80 provides three values: a44-bit address addr(20:63) involved in the transfer; the master requestidentifier req_tag(0:15) that is unique to each master; and the transfertype TType(0:7) which indicates the type of operation, e.g., load,read-with-intent-to-modify (RWITM), DClaim, kill, direct memory access(DMA) read, DMA write, etc. Sector request signal 82 is a 4-bit fieldreq_sector_select(0:3) that indicates which 32-byte sector of the128-byte line is being requested. Snoop response bus 72 has two signals.A snooper response sresp(0:3) is provided on signal 84, e.g., null(clean), retry, go_modified, go_shared, etc. Sector service signal 86 isa 4-bit field sresp_sector_select(0:3) that indicates which 32-bytesector of the 128-byte line is being serviced by this snooper. Combinedresponse bus 74 has two signals. A combined response cresp(0:3) isprovided on signal 88, e.g., retry, go_modified, go_shared, etc.Combined sector signal 90 is a 4-bit field indicating the combinedresults of all the sresp_sector_select(0:3) signals. Data bus 76 hasthree signals. Valid signal 92 indicates that data is valid during thiscycle on the system bus. Data/tag signal 94 provides two values: the tagdata_tag(0:15) which matches the master's tag sent with the initialrequest; and the data, data(0:63). Data sector signal 96 indicates which32-byte sector this data represents.

The control logic 100 of RC machine 52 is adapted to handle 32-bytesystem bus requests. For example, RC machine 52 is able to perform aDClaim or RWITM request on the system bus based on the size of the storefrom store gathering station 102, and the results of the four coherencystates. A 4-bit sector select register 104 is used to indicate which32-byte sectors are involved in the store (or load) request from core42. If more that one 32-byte sector is involved in the transfer (allfour sectors could be involved), then multiple bits in sector selectregister 104 are set.

Various commands from RC machine 52 to system bus 68 may be provided tosupport sector requests. In the event that a store operation is directedto a 128-byte line and all of the 32-byte sectors involved are alreadyin L2 cache 44 in the modified state, then no system bus action isrequired at all (the store can be committed immediately to the L2cache). For a DClaim operation, if any of the four sector bits indicatethat a sector of the relevant line is shared, then RC machine 52 issuesa system bus code for the DClaim operation and sets thereq_sector_select(0:3) field to indicate which sectors the RC machine isrequesting for the operation (one or more of these bits may be set). Fora RWITM operation, if any of the four sector bits indicate that a sectoris invalid, then RC machine 52 issues a system bus code for the RWITMoperation and sets the req_sector_select(0:3) field to indicate whichsectors the RC machine is requesting for the operation (one or more ofthese bits may again be set). For a load operation, RC machine 52 mayask for all 128 bytes, by setting all four of the req_sector_select(0:3)bits. Nevertheless, RC machine 52 may ask for a subset of the line, forany reason, such as a LARX (load-and-reserve) request involved inacquiring a lock variable, which usually has a granularity smaller thanthe full cache line. RC machine 52 can also perform a traditionalload/RWITM/DClaim (of the full 128-byte line) by simply turning on allof the sector select bits in the system bus command.

Each cache also includes a snooper 50 which is adapted to handle the32-byte system bus requests. Snooper machine 50 accepts aload/RWITM/DClaim request from system bus 68 based onreq_sector_select(0:3) which was sent by another L2 master RC machine(i.e., of a processing unit other than processing unit 40). The controllogic 108 of snoop machine 50 also includes a 4-bit sector selectregister 110. Because a given snooper may only have a portion (e.g., 32bytes) of the line being requested, that snooper may only be able tointervene with that portion of the cache line. The snooper usesdata_sector_select(0:3) to identify which portion it is returning. It istherefore possible for a master to request a load of all 128 bytes of acache line and have four different snoopers deliver the four differentsectors that they respectively have cached in a modified state.

Main memory can supply an incomplete portion of the line as well. Forexample, a first sector of a line may be modified in a first cache wherethe remaining sectors are invalid, and a second sector of the line maybe modified in a second cache where the remaining sectors are likewiseinvalid, with no other caches having any valid copies of any of thesectors. In this scenario, the first and second caches will intervenewith their respective sectors, and memory controller 106 will determinethat the remaining two sectors need to be sourced by main memory.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that such modifications can bemade without departing from the spirit or scope of the present inventionas defined in the appended claims.

What is claimed is:
 1. A method of maintaining cache coherency in amultiprocessor computer system having a plurality of processing units,each processing unit having at least one cache memory containing aplurality of sectored cache lines, comprising the steps of: issuing arequest from a first cache memory to retrieve an entire cache line; andin response to the issued request, sourcing only a first sector of therequested cache line from a second cache memory to the first cachememory, and sourcing only a second sector of the requested cache linefrom another memory device to the first cache memory.
 2. The method ofclaim 1 further comprising the steps of: snooping the issued request bythe second cache memory; and transmitting a snoop response by the secondcache memory to indicate that the second cache memory will source thefirst sector of the requested cache line to the first cache memory. 3.The method of claim 1 wherein the step of sourcing only the secondsector of the requested cache line further comprising sourcing only thesecond sector of the requested cache line from a third cache memory tothe first cache memory, in response to the issued request.
 4. The methodof claim 1 wherein the step of sourcing only the second sector of therequested cache line further comprising sourcing only the second sectorof the cache line from a system memory device of the multiprocessorcomputer system to the first cache memory, in response to the issuedrequest.
 5. The method of claim 4 further comprising the steps of:snooping the issued request by the second cache memory; and transmittinga snoop response by the second cache memory to indicate that the secondcache memory will source the first sector of the requested cache line tothe first cache memory; snooping the issued request by a third cachememory; transmitting a second snoop response by the third cache memoryto indicate that the third cache memory will source the second sector ofthe requested cache line to the first cache memory; receiving the snoopresponses from the second and third cache memories by a system memorydevice; and in response to the received snoop responses, transmitting acombined response by the system memory device to indicate that thesystem memory device will source the third sector of the requested cacheline to the first cache memory.
 6. The method of claim 1 wherein saidissuing step sets a bit as part of the request indicating a specificdemand for the first sector of the requested cache line.
 7. The methodof claim 1 further comprising the steps of: assigning a first cachecoherency state to the first sector in the second cache memory; andassigning a second cache coherency state, different from the first cachecoherency state, to the requested cache line in the second cache memorywhile maintaining the first cache coherency state for the first sectorof the requested cache line in the second cache memory.
 8. A computersystem comprising: a system memory device; a plurality of processingunits each having a cache memory containing a plurality of sectoredcache lines; bus means for interconnecting the system memory device andthe processing units; and intervention means for issuing a request froma first cache memory to retrieve an entire cache line, and for sourcingonly a first sector of the requested cache line from a second cachememory to the first cache memory and for sourcing only a second sectorof the request cache line from another memory device to the first cachememory, in response to the issued request.
 9. The computer system ofclaim 8 wherein the second cache memory includes means for snooping theissued request, and means for transmitting a snoop response indicatingthat the second cache memory will source the first sector of therequested cache line to the first cache memory.
 10. The computer systemof claim 8 wherein the another memory device is a third cache memory.11. The computer system of claim 8 wherein the another memory device isa system memory device.
 12. The computer system of claim 8 wherein: thesecond cache memory includes means for snooping the issued request andtransmitting a first snoop response indicating that the second cachememory will source the first sector of the requested cache line to thefirst cache memory; a third cache memory includes means for snooping theissued request and transmitting a second snoop response indicating thatthe third cache memory will source the first sector of the requestedcache line to the first cache memory; and a system memory deviceincludes means for receiving the snoop responses from the second andthird cache memories and transmitting a combined response indicatingthat the system memory device will source the third sector of therequested cache line to the first cache memory.
 13. The computer systemof claim 8 wherein the intervention means issues the request with a bitset to indicate a specific demand for the first sector of the requestedcache line.
 14. The computer system of claim 8 further comprising cachecoherency means for assigning a first cache coherency state to the firstsector in the second cache memory and assigning a second cache coherencystate, different from the first cache coherency state, to the cache linein the second cache memory while maintaining the first cache coherencystate for the first sector of the cache line in the second cache memory.